JPH0621573A - Semiconductor laser and its manufacture - Google Patents

Abstract

(57) [Abstract] [Purpose] An object of the present invention is to provide a DFB laser with increased diffraction efficiency, reduced threshold current and improved output power, and a manufacturing method thereof. [Structure] On an n-type InP substrate 11, an n-type InP buffer layer 12 is interposed and an n-type InGaAsP lower waveguide layer 13, i
-Type InGaAsP active layer 14, p-type InGaAsP upper waveguide layer 15, and p-type InP clad layer 16 are sequentially formed, and the lower diffraction grating 20 is formed at the interface between the lower waveguide layer 13 and the buffer layer 12, and the upper waveguide layer 15 is formed. An upper diffraction grating 21 is formed at the interface of the clad layer 18. The lower diffraction grating 20 is formed by anisotropic etching of the buffer layer 12 using the mask material 23 as an etching resistant mask,
The upper diffraction grating 21 is formed by selectively growing the waveguide layer 13, the active layer 14, and the waveguide layer 15 while leaving the mask material 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser and a manufacturing method thereof, and more particularly to a distributed feedback laser (DFB laser) effective for optical communication and optical measurement and a manufacturing method thereof.

[0002]

2. Description of the Related Art InP-based DFB lasers are indispensable as a light source for a 1.5 .mu.m long-wavelength optical fiber communication system. The DFB laser uses a diffraction grating as a reflecting mirror of a resonator of a semiconductor laser to enable single longitudinal mode oscillation. If a DFB laser is used as the light source and a silica single mode fiber with a minimum loss wavelength band of 1.5 μm is used to construct an optical communication system, chromatic dispersion can be suppressed to a small level, and long-distance, large-capacity optical communication is possible. Becomes

A conventional DFB laser is constructed as shown in FIGS. 5 (a) to 5 (c). FIG. 5A shows an n-type InP substrate 1 with an n-type InP buffer layer 2 interposed therebetween.
P waveguide layer 3a, InGaAsP active layer 4, p-type InP
The clad layer 5 is sequentially laminated, and the diffraction grating 6 is formed between the InP buffer layer 2 and the InGaAsP waveguide layer 3a. 5 (b), the p-type InGaAsP waveguide layer 3b is formed on the active layer 3 contrary to FIG. 5 (a), and the diffraction grating 6 is formed between the InGaAsP waveguide layer 3b and the cladding layer 5. Has been done. In FIG. 5C, InGaAsP waveguide layers 3 a and 3 b are formed below and above the active layer 4, and a diffraction grating 6 is formed between the upper InGaAsP waveguide layer 3 b and the cladding layer 5.

[0004]

In such a DFB laser, since stimulated emission is caused by distributed feedback of light utilizing diffraction, the threshold current and output power of laser oscillation are the feedback efficiency (that is, Diffraction efficiency). However, the conventional structure has a problem that it is difficult to obtain a sufficient feedback efficiency because the diffraction grating is formed on either the upper or lower side of the active layer. The present invention has been made in consideration of such circumstances, and provides a DFB laser capable of increasing the diffraction efficiency and decreasing the oscillation threshold current to obtain a large laser light output, and a manufacturing method thereof. With the goal.

[0005]

In the DFB laser according to the present invention, a lower waveguide layer, an active layer, an upper waveguide layer and a cladding layer are sequentially laminated on a semiconductor substrate via a buffer layer, and the lower waveguide layer is formed. A lower diffraction grating is formed at the interface between the waveguide layer and the buffer layer, and an upper diffraction grating is formed at the interface between the upper waveguide layer and the cladding layer. A method of manufacturing a DFB laser according to the present invention comprises a step of forming a buffer layer on a semiconductor substrate, patterning a mask material at predetermined intervals on the surface of the buffer layer, and using the mask material as an etching resistant mask to form the buffer layer. The layer surface is etched by the anisotropic etching method,
A step of forming a lower diffraction grating, and using the mask material as a mask for selective growth on the buffer layer having the diffraction grating formed thereon, a lower waveguide layer and an active layer are sequentially grown to form a flat surface. And a step of growing the upper waveguide layer on the surface of which an upper diffraction grating is formed by using the mask material as a selective growth mask on the active layer,
A step of growing and forming a clad layer on the upper waveguide layer.

[0006]

In the DFB laser according to the present invention, by providing the diffraction grating above and below the active layer, a high diffraction efficiency can be obtained, the oscillation threshold current can be lowered, and a large laser output light power can be obtained. According to the manufacturing method of the present invention,
Since the mask material used as an etching resistant mask when forming the lower diffraction grating is used as it is as a mask for the subsequent selective growth, the upper and lower diffraction gratings are in a state where the lattice constants are perfectly matched and the phase is the same. Can be formed in a state of being shifted by 180 °. Thereby, excellent single longitudinal mode laser light can be obtained. In addition, by selectively growing the lower waveguide layer and the active layer and flattening them while leaving the mask material, and further selectively growing the upper waveguide layer, an etching process is not required on the surface of the upper waveguide layer. An upper diffraction grating is formed on the.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. 1 (a) and 1 (b) show a D according to an embodiment of the present invention.
FIG. 3 is a perspective view showing a structure of an FB laser and a cross-sectional view of a main part along the optical axis thereof. An n-type InP buffer layer 12 is formed on an n-type InP substrate 11, and an n-type InGaAsP lower waveguide layer 13, an i-type InGaAsP active layer 14, and a p-type InGaAsP upper waveguide layer 15 are sequentially stacked on the n-type InP buffer layer 12. There is. The composition of the InGaAsP active layer 14 is selected so as to match the oscillation wavelength. The upper and lower InGaAsP waveguide layers 13 and 15 are semiconductor materials having a forbidden band width larger than that of the InGaAsP active layer 14, and their composition ratio relationship is set so as to have a refractive index larger than that of the InP buffer layer 12.

A p-type InP clad layer 18 is formed on the upper waveguide layer 15, and I of high impurity concentration is further formed thereon.
An nGaAs cap layer 19 is formed. Each of these semiconductor layers is etched in a reverse mesa shape to a depth reaching the InP buffer layer 12 to form a stripe pattern, and the p-type InP layer 16 and the n-type InP layer are formed around the stripe pattern as current blocking layers. Layer 1
7 is embedded. Electrodes are formed on the front surface of the cap layer 19 and the back surface of the substrate 11 although not shown in the drawing.

As shown in FIG. 1B, a lower diffraction grating 20 is provided below the active layer 14, and an upper diffraction grating 2 is provided above the active layer 14.
1 are formed respectively. That is, the lower diffraction grating 20
Is formed at the interface between the InP buffer layer 12 and the lower waveguide layer 13, and the upper diffraction grating 21 is formed at the interface between the upper waveguide layer 15 and the cladding layer 18.

Reference numeral 23 in FIG. 1 (b) is an etching resistant mask for selectively etching the buffer layer 12 to form the lower diffraction grating 20, and is also a mask material used for the subsequent selective growth. . The mask material 23 is an insulating film such as SiO ₂ , Si ₃ N ₄ or the like, or a semiconductor film which is transparent at the oscillation wavelength of the laser. When a semiconductor is used as the mask material 23, in order for the selective liquid phase growth to be effective, the deviation of the lattice constant from the material exposed in the mask opening is 0.2.
% Or more, and in such a range, for example, In
GaAs, InGaAsP, etc. can be used as a mask material.

The manufacturing process of the DFB laser of this embodiment is as follows.
A description will be given below with reference to FIGS. 2 and 3. As shown in FIG. 2A, n-type InP is formed on the (100) InP substrate 11.
The buffer layer 12 is grown epitaxially and then this I
A mask material 23 is formed on the nP buffer layer 12. Then, by using photolithography, the mask material 23 is patterned into a stripe lattice pattern as shown in FIG. Since the mask material 23 is left inside the element as it is, the material, the film thickness, the pattern width, etc. are selected so that the crystal lattice defects at the time of light absorption and regrowth do not become a problem.
For example, SiO ₂ is used, and both the film thickness and the width are set to about several tens nm or less. When InGaAs is used, the film thickness is set to about several nm or less so that light absorption does not become a problem. Since the pitch of the mask material 23 becomes a lattice constant,
In consideration of the difference in refractive index between the InP buffer layer 12 and the waveguide layer formed thereafter, the selection is made so as to obtain the required oscillation wavelength.

Next, as shown in FIG. 2C, the mask material 23
Is used as an etching-resistant mask, the InP buffer layer 12 is mesa-etched by anisotropic etching,
The lower diffraction grating 20 is formed by a triangular groove whose (111) plane is exposed. Then, as shown in FIG. 2D, the n-type InGaAsP lower waveguide layer 13 is selectively grown by the vapor phase epitaxial growth method using the mask material 23 as it is as a selective growth mask. Further, as shown in FIG. 3A, the i-type InGaAsP active layer 14 is selectively grown by the vapor phase epitaxial growth method,
Flatten the surface. The InGaAsP waveguide layer 13 is In
A semiconductor material having a forbidden band width larger than that of the GaAsP active layer 14 and their composition ratios are selected so as to have a larger refractive index than the InP buffer layer 12.

Next, as shown in FIG. 3B, the mask material 2
3 is used as a mask for selective growth, and the InGaAsP upper waveguide layer 15 is further formed by a vapor phase epitaxial growth method.
Grow. At this time, since the InGaAsP active layer 14 is flattened and the (100) plane is exposed on the surface and growth does not occur on the mask material 23, the upper waveguide having a triangular cross section is formed on the active layer 14. The layer 15 is formed, and the upper diffraction grating 21 is automatically formed on the surface thereof.

After that, as shown in FIG. 3C, p-type In
The P clad layer 18 and the InGaAs cap layer 19 are formed. These also grow on the mask material 23 and the surface is flattened. Then, these semiconductor layers are patterned in a stripe pattern, and a p-type InP layer and an n-type InP layer to be a current blocking layer are formed outside the stripe pattern.
Epitaxially grow the layer. Finally, although not shown, electrodes are formed on both surfaces to complete the device.

In the DFB laser of this embodiment, a diffraction grating as a distributed reflector is formed above and below the active layer.
Moreover, the upper and lower diffraction gratings are formed as a 180 ° inversion pattern which is perfectly matched by the mask material, and contributes as feedback light for laser oscillation. The first-order diffracted light by each diffraction grating is perfectly aligned in phase. Will be things.
Therefore, the diffraction efficiency is almost doubled as compared with the conventional case where there is one diffraction grating. As a result, the threshold current for laser oscillation is reduced by about 30% as compared with the case where there is one diffraction grating, and as a result, the laser output light power at the same current density is improved.

FIG. 4 shows a sectional structure of a main part of a DFB laser according to another embodiment of the present invention, corresponding to FIG. 1 (b). The basic structure and manufacturing process are similar to those of the previous embodiment, and therefore, parts corresponding to those of the previous embodiment are designated by the same reference numerals. The difference from the previous embodiment is that in this embodiment, the active layer 141 is already formed before the mask material 23 is formed. The active layer 141 and the buffer layer 12 thereunder are etched using the mask material 23 to form the lower diffraction grating 20. After that, the lower waveguide layer 13, the active layer 142, the upper waveguide layer 15, and the cladding layer 18 are formed in the same manner as in the previous embodiment.

In the structure of the previous embodiment, the active layer 14 is separated by the mask material 23 and becomes scattered, whereas in this embodiment, the active layers 141 and 142 are continuous and alternately arranged. Becomes Therefore, even if the mask material 23 absorbs light to some extent, its influence is eliminated and higher efficiency can be obtained.

[0018]

As described above, the DFB according to the present invention
In the laser, by providing the diffraction grating above and below the active layer, a high reflection efficiency is obtained, the oscillation threshold current is lowered, and a large laser output light power is obtained. Further, according to the method of the present invention, the mask material used as an etching resistant mask when forming the lower diffraction grating is used as a mask for the subsequent selective growth, and the upper and lower diffraction gratings have completely the same lattice constant. It is formed with the reversed pattern. Thereby, excellent single longitudinal mode laser light can be obtained. Further, by repeating the crystal growth with the mask material left, the upper diffraction grating is formed on the surface of the upper waveguide layer without requiring an etching step.

[Brief description of drawings]

FIG. 1 is a perspective view and a sectional view of an essential part of a DFB laser according to an embodiment of the present invention.

FIG. 2 is a diagram showing a manufacturing process of the embodiment.

FIG. 3 is a diagram showing a manufacturing process of the embodiment.

FIG. 4 is a sectional view showing the structure of a main part of a DFB laser according to another embodiment of the present invention.

FIG. 5 is a cross-sectional structural diagram of a conventional DFB laser.

[Explanation of symbols]

11 ... n-type InP substrate, 12 ... n-type InP buffer layer,
13 ... n-type InGaAsP lower waveguide layer, 14 ... i-type I
nGaAsP active layer, 15 ... p-type InGaAsP upper waveguide layer, 16 ... p-type InP layer, 17 ... n-type InP layer, 1
8 ... p-type InP clad layer, 19 ... InGaAs cap layer, 20 ... lower diffraction grating, 21 ... upper diffraction grating, 23
… Mask material.

Claims (2)

[Claims] 1. A lower waveguide layer, an active layer, an upper waveguide layer and a clad layer are sequentially laminated on a semiconductor substrate with a buffer layer interposed therebetween, and a lower diffraction grating is formed at an interface between the lower waveguide layer and the buffer layer. A semiconductor laser, wherein an upper diffraction grating is formed at an interface between the upper waveguide layer and the cladding layer. 2. A step of forming a buffer layer on a semiconductor substrate, patterning a mask material at a predetermined interval on the surface of the buffer layer, and using the mask material as an etching resistant mask, the surface of the buffer layer is anisotropic. Etching by an etching method to form a lower diffraction grating, and a lower waveguide layer and an active layer are sequentially grown on the buffer layer on which the diffraction grating is formed by using the mask material as a mask for selective growth. And then planarizing the surface, using the mask material as a selective growth mask on the active layer, growing and forming an upper waveguide layer having an upper diffraction grating formed on the surface, and the upper conductive layer. And a step of growing and forming a clad layer on the waveguide layer.